The use of a current to aid in the sintering of ceramic materials has been published in a large number of papers.
Electric pulse assisted consolidation (EPAC) includes all processes based on heating the material to be sintered with a pulsed DC current. Other names for this technique are spark plasma sintering (SPS), pulsed electric current sintering (PECS), field assisted sintering technique (FAST), plasma-assisted sintering (PAS) and plasma pressure compaction (P2C). These techniques will hereafter be referred to as SPS.
SPS is a relatively new sintering technique, even though the idea to compact metallic materials by an electro-discharge process existed already in the 1960s (U.S. Pat. No. 3,241,956). Electrical energy pulses are applied to a powder which is placed in a die between conducting punches. The sintering method allows production of fully dense materials within minutes while applying high heating rates and short dwell times. A pulsed DC current with typical pulse durations of a few ms and currents of 0.5-30 kA flows through the punches, the die and, depending on the electrical properties of the specimen, also through the specimen. The electrical pulses are generated in the form of pulse packages where the on:off relation is in the region of 1:99 to 99:1. The pressure is directly applied on the punches in a uniaxial direction, and thereby on the powder bed, and is typically between 5 and 200 MPa. The technique is today used to compact a variety of different metallic and ceramic materials.
Using conventional sintering methods like hot pressing (HP), hot isostatic pressing (HIP) or pressure-less sintering, the densification of ceramic materials such as silicon nitride and sialon (silicon nitride based material where silicon and nitrogen are substituted with aluminium and oxygen) is accompanied by an uncontrolled growth of the crystalline grains. The normal sintering practice, both for conventional sintering methods and for SPS, is often to heat the powder to be sintered at a certain rate and thereafter holding it at a certain temperature and pressure until the maximum density is reached. The grain size is then increasing with the density. The uniaxial pressure leads to an anisotropic microstructure of the ceramics, as the grain growth is favoured perpendicularly to the pressure. The orientation and dimensions of the crystalline grains of the ceramics have a large impact on the mechanical properties of the components. A lot of effort has therefore been focused on attempts to tailor the grain sizes and microstructures of the ceramics in order to obtain desired mechanical properties of the resulting components.
It is known to use SPS for sintering ceramics with very fine grain structure. In U.S. Pat. No. 5,622,905 a sintered silicon nitride body is described having a uniform fine crystal size not exceeding 200 nm after sintering by spark plasma sintering, microwave sintering, ultra-high pressure sintering, or the like.
Several publications report on sintering of silicon nitride and sialons with SPS. In “Formation of in situ reinforced microstructures in alfa-sialon ceramics: Part III; Static and dynamic ripening”, J. Mater. Res. Vol. 19, No 8, August 2004, pages 2402-2409, is described how (YB+Y)-stabilized alfa-sialon samples are consolidated by spark plasma sintering. It was found that below a temperature threshold consolidation of alfa-sialon could be achieved with very limited grain growth. A temperature threshold for grain growth is also observed and annealed consolidated samples were showing elongated alfa grains.